Dry dock for south Dakota class battleship

I would use particle board for basic structure, then sheath it in sheet styrene.  Wood might be a bit better but more expensive depending on size of wood. And, particle board less likely to warp.

Try pink insulation foam board. Its robust, lightweight, cuts easy on a table saw and doesn't warp. I built this diorama from the pink stuff, skim coated it with joint compound and used a nail and ruler to make the expansion joints in the "concrete". When dry I airbrushed flat light gray and shaded the floor and walls by brush. I'm planning a drydock diorama also with a Flower Class corvette, I plan on using the "pink" stuff, it works for me.....  Hope this helps.

Thanks everyone for the suggestions. I will consider them carefully. I'm leaning towards what Don said above but im still working everything out.